We propose to study the synaptic organization of the neuronal circuits responsible for the generation of hippocampal theta rhythm. This is an ideal model system for the study of generation of rhyathmic slow waves in brain. The mechanisms of normally occuring slow waves must be known before we can hope to understand the mechanisms of slow waves due to various pathological conditions. Knowledge of the mechanisms of theta rhythm may also suggest some hypotheses concerning its function. Since some theta rhythm presumably involves muscarinic cholinergic mechanisms (Kramis, et al, 1975), this represents a unique opporatunity to apply quantitative electrophysiological techniques to elucidate the mechanisms of a normal cholinergically modulated brain "state". We will make intracellular recording farom hippocampal neurons during theta rhythm to assess the contributions of EPSPs an IPSPs. Intracellular a.c. impedance measurements made with since waves at several frequencies will help in judging the relative proximity of the major conductances. We will do current source density studies to localize the laminae with sources and sinks of current related to phase and or presence or absence of theta rhythm. These data will be compared with existing anatomical and electrophysiological data in an attempt to identify the active synapses on the various phases of theta rhythm. If time permits, lesions will be made in regions which appear to be sources of afferents contributing to theta rhythm. Our long range goal is a quantitative model and a description of the cellular interactions foar the generation of theta rhythm.